1. Field of the Invention
The present invention generally relates to switching power supplies. In particular, the present invention is directed to a switching power supply that converts an alternating current (ac) input signal having a wide range of voltages to a direct current (dc) output signal having a substantially constant voltage and/or current.
2. Description of the Prior Art
Switching power supplies have certain properties that make them desirable. For instance, there is very little power dissipation, making switching supplies efficient even if there is a large voltage drop required from input to output. Switching power supplies can also generate output voltages that exceed the input voltage, which enables these supplies to run directly from a rectified power line without requiring an alternating current (ac) power transformer. The result is a compact, lightweight, and efficient power supply that is desirable for universal use in a variety of applications.
Direct current (dc) power supplies must often adapt to wide variations in input voltages. A multiple source power supply may be required in certain applications where the output of the supply depends on the device being supplied. A multiple source power supply may derive its input from multiple ac lines, one or more batteries, or even solar collectors to power the load connected to the output terminals.
Two conventional solutions for implementing multiple input power supplies are provided in FIGS. 1 and 2. As shown in FIG. 1, one such solution uses a transformer 10 to establish electrical connection of a multi-input transformer ac supply to a dc regulator. The primary of the transformer 10 includes several taps 12 to accommodate different values of AC input voltage (VAC1, VAC2, VAC3) and maintain approximately the same value at the secondary voltage VIN. The secondary winding 16 may supply this voltage VIN to a power supply for rectification and generation of the dc power signal. A second solution that provides a similar result is shown in FIG. 2, in which power sources 12 are capacitively coupled to generate an ac voltage that is subsequently input to the power supply or load.
In each of the above configurations, it is necessary that the user manually select the connections for the ac input voltage during installation. Even in applications where power supplies have only two ac input connections, manual selection of the input connections is still required through incorporation of a multi-position switch or “intelligent” power supply having automated voltage selection capability. Since these solutions incorporate a multi-input circuit to accommodate various ac voltages, an undesirable selection must be made between manual setup for a particular voltage or the use of sophisticated and costly automatic voltage selection components.
Additional cost is incurred due to the expense of the purchase, installation, and maintenance of such voltage selection techniques. Inherent risks associated with the selection of an improper voltage can also result in improper functioning of the circuit and possible damage to the system within which it is implemented, thereby incurring additional financial cost and safety hazards.
It is therefore desirable to retain the range of ac input voltages that may be applied to a power supply circuit while minimizing the number of required inputs. Such a power supply circuit advantageously avoids restrictions on ac input voltage values.
As indicated above, some conventional solutions use a step-down transformer to convert the ac input signal to a lower voltage required by load circuitry. When the supply must be able to adapt to more than one ac input voltage, most conventional implementations use transformers with multi-tap inputs and fixed step-down ratios. As disclosed in U.S. Pat. No. 6,563,721, which is incorporated herein by reference, an alternative solution uses a single-tap (two-wire) input transformer in conjunction with a wide voltage-range switching regulator. In this instance, the power supply is essentially a voltage-to-voltage converter. That is, the supply converts a variable ac input voltage from a transformer secondary or battery into a constant dc output voltage required by the load.
As shown in FIG. 3, another solution incorporates a capacitor 18 electrically connected in series with a rectifier bridge 20 and a load 22. The impedance of the capacitor 18 limits the load current ILOAD to a predetermined value, and thus functions as a constant ac current generator. Since the ac input voltage VIN is typically much greater that the dc output load voltage VLOAD, the ac current is fixed and essentially determined by the ac input voltage VIN and the value of capacitor 18. However, it is difficult to achieve high efficiency in such a power supply when using a constant-current ac generator to drive the voltage-to-voltage regulator.
As discussed above, to adapt the capacitor-input power supply to different ac input voltages, such as 120V, 277V, 437V, and the like, the multi-input design shown in FIG. 2 may be used, which requires separate capacitors 24, 26, and 28 and input connections for each ac voltage. However, this implementation increases the material cost, overall dimensions, and the risk of faulty installation.
It is therefore desirable to provide a power supply based on a single capacitor input that converts a wide range of input ac signals to a regulated dc output signal having substantially constant voltage and/or current.